INTRODUCTION
Two thirds of all fish escapes, in terms of both incidents and number of fish, occur through a
hole in the net (Jensen et al. 2010). Holes form due to a number of reasons, the most common
being: biting of the net by predators or the caged fish; abrasion; boat collisions; contact with
flotsam, and cage handling procedures such as lifting.
There are currently no written standards specifying the properties of the materials used in
aquaculture, or explaining how the materials might degrade with time and cumulative wear
and tear. This is particularly problematic for non-metallic materials such as polyethylene,
polyamide and polypropylene; their functional properties are known to change with
temperature conditions and over time.

OBJECTIVE
We aimed to determine the functional characteristics of commonly used aquaculture materials
and establish objective methods for testing the materials used in sea-cage aquaculture nets.

248

METHODS
New and used net materials were collected from a number of net producers and fish farmers.
Unfortunately, the history of the used net samples was generally not known; farmers do not
keep fully detailed records about when they wash their nets (whether in-situ or onshore),
whether or not the nets are coated, or details of any operations performed on the net during
use that could influence the strength of the material. Some variation in the results is, therefore,
to be expected.
Strength reduction of new and used nets, with different mesh sizes and thread numbers, were
compared. A number of factors can reduce the strength of the nets used in fish-farm cages,
such as: onshore washing; coating; in-situ washing using high-pressure disks; creep during
lifting operations; ultra-violet radiation, and abrasion. We tested how some of these factors
would affect net strength.
Nets may also be subject to extra loads from the weights or sinker-tube, which are used to
maintain the shape and volume of the cage, plus any water currents. Continuous loads can
introduce creep effects, especially on plastic materials such as nylon and Dyneema (http://
en.wikipedia.org/wiki/Creep_(deformation)). In addition, waves often introduce motions to the
floating collar that propagate as dynamic tensions in the net. To investigate how this may
affect the break-strength of the net, nylon and Dyneema samples were tested at increasing
rates of deformation.
The effect of onshore machine washing was investigated by washing net samples together with
net cages that had been used to farm salmon for one production cycle. After washing the net
panels 1 â&#x20AC;&#x201C; 5 times the mesh strength was tested. During summer and early autumn months
the net cages, especially in mid- and southern parts of Norway are washed, using high pressure
disks, as often as once per week to remove biofouling. Nylon and Dyneema net samples were
washed using industry standard equipment to investigate the effect on mesh strength.

www.preventescape.eu

249

Testing procedure
Numerous net materials were tested in a uni-axial tensile test machine (Figure 6.4.1). Two
different test types were used to test the strength of the material; mesh strength tests
(Figure 6.4.2) and uni-axial tensile tests to determine the tensile strength of the thread as well
as the elasticity of the material (Figure 6.4.3). Tensile tests were conducted on 50 cm lengths
of net. Pure nylon netting was cut while wet, whereas netting containing Dyneema fibres were
cut while dry, using a butane powered cutting tool, to prevent filaments from slipping through
the knots at the thread ends. Unless otherwise stated, samples were immersed in tap water
at 20 â&#x20AC;&#x201C; 23Â°C for at least 24 h prior to testing. Nylon and Dyneema specimens were subjected
to a permanent load for 30 minutes. After seven days of relaxation the break-strength of the
specimen was determined to investigate whether the 30 minutes creep-load had weakened
the material. Quasi-static mesh-strength tests (ISO 1806:2002; ISO 2002) were performed on
new and used material to serve as a reference for other tests. Equivalent test settings were
adopted for the uni-axial tensile tests.
Test methods were developed to investigate the influence from washing in-situ with high
pressure cleaning disks (Figure 6.4.4), and load duration â&#x20AC;&#x201C; both creep and dynamic tests. Two
different nylon materials (N10 and N11), with (C) and without coating (U), as well as coated
Dyneema (N12_C) net were subjected to one hour of controlled abrasion (Figure 6.4.5), and
the strength of the material tested using uniaxial tensile tests.

Net type
Tests were performed on samples of the type of new materials that are supplied to the
Spanish fish farming industry (Figure 6.4.6), and on used net samples from European seabass
(Dicentrarchus labrax) and gilthead seabream (Sparus aurata) cages (Figure 6.4.7). A 30%
decrease in the break-load and a 15 â&#x20AC;&#x201C; 40% decrease in the length of elongation at rupture were
found in nets that had been used for one production cycle (Figure 6.4.6). The Dyneema nets were
found to have much higher break-load and stiffness compared to the nylon nets. The difference
between nets from different producers was relatively small. The mesh strength of nets used to
farm Atlantic cod (Gadus morhua) and Atlantic salmon (Salmo salar) were determined (Figures
6.4.8, 6.4.9 and Figure 6.4.10). There was no obvious correlation between production year or
the number of wash cycles and degradation in the strength of the net.
Figure 6.4.6. Mesh strength and
elongation at break - virgin and used
Spanish net materials.

252

Figure 6.4.7. Mesh strength and elongation at rupture - new nets from different producers.

Figure 6.4.8. Mesh strength for nets of varying age.

www.preventescape.eu

253

Figure 6.4.9. Changes in net strength
due to washing of the net.

Figure 6.4.10. Force vs strain curve for
different net materials.

254

Creep loads
Tests showed that a 30 minute creep-load had a positive effect on the break-load for the
Dyneema material (N6; Figure 6.4.11). For the C90 case, (i.e. where the creep-load is equal
to 90% of the break-load), the break-strength increased by close to 20%. An opposite effect
was observed for nylon nets (N7 and N8) where the break-strength was reduced by 5 â&#x20AC;&#x201C; 10%.
Figure 6.4.11. Stress at break divided by
stress at break for virgin material.

MESH

STRENGTH AND DEFORMATION RATES

The relative mesh strength, normalized against the mesh strength reported in ISO 1806:2002
(ISO 2002), of one of the two nylon nets tested, increased by 10% as the rate of deformation
increased (Figure 6.4.12). However, no significant change was detected for the second nylon
sample and the Dyneema mesh strength was reduced by up to 10%. The highest rates of
deformation tested gave a total load time of 1.2 second for the Dyneema net, 1.6 seconds
for one of the nylon nets, and 2.7 seconds for the other. These values are within the range of

www.preventescape.eu

255

loads expected due to wave action. Snap loads will have even shorter load times. These results
indicate that short periods of extra load bearing, such as from waves, should not be a problem
for nylon nets, but suggest that for Dyneema nets these loads may be problematic, particularly
if snap loads with even higher rates of deformation occur.

Figure 6.4.12. The effect of rate of deformation on relative mesh strength.

WASHING
Washing of nets led to a distinct drop in strength after only one wash (Figure 6.4.13). A reduction
in strength of 20% was observed after the net panels had been washed four times. According to
the Norwegian technical standard NS9415 (Standard Norge 2009) net cages must be discarded
if the remaining strength is reduced by 35%. Net washing therefore accounts for more than half
of the allowable strength reduction. In comparison, there was no reduction in strength even after
40 wash cycles of in-situ high pressure cleaning (Figure 6.4.14). However, escape incidents have
occurred during high pressure cleaning, especially in connection with the use of cranes, but these
incidents have occurred due to the incorrect use of equipment by their operators.

256

Figure 6.4.13. How the washing of the
net effect the mesh strength.

Figure 6.4.14. Effect of high pressure claning on the mesh strength.

www.preventescape.eu

257

ABRASION
Following abrasion testing, the reduction in strength was significant for all of the tested nets
(Figure 6.4.15). The coating had a positive effect on the abrasion resistance of nylon nets.
Even though the tensile strength of new Dyneema net is higher than that of nylon, the relative
reduction following abrasion was far larger than for the nylon nets. This result may reflect the
fewer filaments in the Dyneema thread.

Figure 6.4.15. The effect on abrasion on tensile strength.

RECOMMENDATIONS
s Used net materials showed a significant reduction in strength, even after just one production
cycle. However, no clear relationship was found between the duration of net use and the
subsequent reduction in strength.
s High pressure cleaning had no effect on mesh strength.
s Cleaning of nets in washing machines lowered the strength of the nets by up to 20% after 4
wash cycles.
s Abrasion significantly reduced the strength of the nets. Coating limited the reduction in
strength, but could not eliminate it.